No, and it is worth being precise about where the protection ends. Telegram secures data in transit and communication between people. It cannot secure a device that somebody else is holding.
What it does protect against
Everything you send and receive, including media and files, cannot be deciphered when intercepted by your internet service provider, by the owner of a Wi-Fi network you connect to, or by any other party sitting on the network between you and the service. That covers the entire class of attacks that involve watching traffic.
What it cannot protect against
- Someone picking up your unlocked phone. No amount of encryption applies once the chats are already open on screen.
- Someone watching over your shoulder, or photographing the screen with another device.
- A compromised device. Malware or root access sits inside, past the point where encryption is relevant.
- You being persuaded to hand something over. A login code given to a convincing stranger defeats every technical control at once.
The part you control
- A passcode on the app, separate from the device lock, so an unlocked phone does not mean open chats.
- Two-step verification, so an intercepted SMS code is not enough to take the account.
- Reviewing Settings, Devices and terminating sessions you do not recognise.
- Never sharing a login code with anyone, for any stated reason.
In practice, accounts are lost to a shared code or an unlocked device far more often than to anything involving cryptography. The unglamorous controls are the ones that matter.
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